1. Field of the Invention
The present invention relates to an apparatus and method for preventing an overshoot in the rotation speed of an internal-combustion engine.
2. Description of the Related Art
It is well known that an internal-combustion engine is started by rotating the crank shaft of the engine by means of a starter that is coupled to the engine. In general, the rotation speed of an internal-combustion engine is increased to 200 to 300 rpm at the time of a start and is thereafter increased further to a self-ignition rotation speed of the engine, that is, an idling rotation speed or a rotation speed of a control by a controller for the engine.
In starting an internal-combustion engine, to improve the ignition performance in a range lower than an idling rotation speed, the controller for the engine performs a control of promoting a self-ignition-based operation by controlling the ignition timing and increasing a fuel supply rate (e.g., refer to JP-A-2002-191192 (paragraph 0002)).
In conventional internal-combustion engines that are started in the above-described manner, the following phenomenon occurs at a start that is associated with an increase in fuel supply rate. At an instant when self-ignition of the engine has just started, because of mechanical delays in the air system and the fuel system of the engine and delays in the control system that detects a rotation speed of the engine and issues instructions relating to an air supply rate, a fuel injection amount, and ignition timing in accordance with the detected rotation speed, torque is generated that is higher than necessitated to attain an idling rotation speed or a target rotation speed of an instruction from the controller of the engine, resulting in an overshoot in rotation speed.
Such an overshoot in the rotation speed of the engine causes a steep rise in rotation speed particularly when the engine is restarted in an idling-stop state. As a result, the pressure of oil as a medium of a coupling portion between the transmission and the engine increases sharply to possibly cause an abrupt start of the vehicle or give a shock to it. This is problematic in terms of the safety and comfort of a drive.
To solve this problem, the above-mentioned patent document, for example, proposes a control method in which a deviation of an actual rotation speed of an internal-combustion engine from a target one is calculated and the torque of a starter/battery charger is switched to the positive or negative side in accordance with the calculated deviation. However, although an attempt to prevent an overshoot in the engine should be performed instantaneously, in the method of this document the overshoot-preventive control takes considerable time, that is, cannot be performed instantaneously. Further, this method makes it necessary to recognize a torque generation characteristic of the engine that depends on its state and a toque characteristic involving the engine and the starter/battery charger. As such, this method has an aspect that complicated calibration is necessary.